Moine mylonite, Moine Thrust Zone

Locality: Knockan Crag

Outcrop

Moine mylonite, Moine Thrust Zone, Knockan Crag
The rock has a very well developed platy texture, and splits into thin flags

Moine mylonite, Moine Thrust Zone, Knockan Crag
Another view of the same rock face, showing the intense fine banding in the mylonite.

Hand specimen

Moine mylonite, Moine Thrust Zone, Knockan Crag
This is a view of a cleavage surface of the mylonite. It looks rather like a slate, and the surface has a striation on it that shows the direction of movement of the thrust sheet.

Moine mylonite, Moine Thrust Zone, Knockan Crag
This oblique view shows, particularly on the left-hand weathered edge of the sample, its tendency to split into sheets

Thin section

Moine mylonite, Moine Thrust Zone, Knockan Crag
The cleavage of the mylonite is due to the aligned flakes of mica (here they have a yellowish colour). At the microscopic scale the banding is not perfectly flat, as it wraps around more resistant crystals.

Plane polarized light. Field of view 2.5 mm across.

Moine mylonite, Moine Thrust Zone, Knockan Crag
Between crossed polars the crystals can be seen more clearly. When the shearing occurred, the rock was quite hot (probably about 300 degrees Celsius). The quartz was able to change shape and form new smaller crystals, allowing the rock to flow and become a mylonite. The larger crystals are feldspar, which does not recrystallize as easily as quartz when sheared. The bright coloured flakes are mica.

Crossed polars. Field of view 2.5 mm across.

D.J. Waters, Department of Earth Sciences, May 2003